1. Field of the Invention
The present invention relates to an optical tomograph that obtains optical tomographic images by OCT (Optical Coherence Tomography) measurement.
2. Description of the Related Art
Conventionally, optical tomographs that utilize OCT measurement are used to obtain optical tomographic images of living tissue. The optical tomographs are applied to obtain tomographic images of the fundus, the anterior ocular segment, and skin. Other applications of the optical tomographs include observation of arteries employing fiber probes, and observation of digestive organs by inserting fiber probes through forceps channels of endoscopes. In these optical tomographs, a low coherence light beam emitted from a light source is divided in to a measuring light beam and a reference light beam. Thereafter, a reflected light beam, which is the measuring light beam reflected or backscattered by a measurement target when the measuring light beam is irradiated onto the measurement target, is combined with the reference light beam. Tomographic images are obtained, based on the intensity of a coherent light beam obtained by combining the reflected light beam and the reference light beam.
OCT measurement can be roughly divided into two types, TD-OCT (Time Domain Optical Coherence Tomography) and FD-OCT (Fourier Domain Optical Coherence Tomography). In TD-OCT measurement, the intensity of the interference light beam is measured while changing the optical path length of the reference light beam. Thereby, intensity distributions of the reflected light beam corresponding to measuring positions in the depth direction of the measurement target (hereinafter, referred to as “depth positions”) are obtained.
On the other hand, in FD-OCT measurement, the optical path lengths of the reference light beam and the signal light beam are not changed. The intensity of the interference light beam is measured for each spectral component thereof, and frequency analysis, such as Fourier transform, is administered on the obtained spectral interference intensity signals. Thereby, intensity distributions of the reflected light beam corresponding to the depth positions of the measurement target are obtained. FD-OCT measurement has been gathering attention recently as a method that enables high speed measurement, due to the mechanical scanning associated with TD-OCT measurement being obviated.
Optical tomographs that perform SD-OCT (Spectral Domain Optical Coherence Tomography) measurement and optical tomographs that perform SS-OCT (Swept Source Optical Coherence Tomography) measurement are two types of optical tomographs that employ FD-OCT measurement. In an SD-OCT optical tomograph, a wide band low coherence light beam is emitted from an SLD (Super Luminescent Diode), an ASE (Amplified Spontaneous Emission) light source, or a white light source. The wide band low coherence light beam is divided into a measuring light beam and a reference light beam by a Michelson interferometer or the like. Thereafter, the measuring light beam is irradiated onto a measurement target, and a reflected light beam reflected by the measurement target is caused to interfere with the reference light beam. The interference light beam formed thereby is spectrally decomposed into each frequency component by a spectrometer, and the intensity of each frequency component of the interference light beam is measured by a detector array, in which elements such as photodiodes are provided in an array. A computer administers Fourier transform on the obtained spectral interference intensity signals, to obtain a tomographic image (refer to U.S. Patent Application Publication No. 20050018201). U.S. Patent Application Publication No. 20050018201 further discloses a method in which the interference light beam is spectrally decomposed into spectral bands, and the spectrally decomposed interference light beams are respectively detected by separate photodetectors, in order to improve detection accuracy.
Further, U.S. Patent Application Publication No. 20050018201 proposes an optical tomograph that utilizes SS-OCT measurement, as an apparatus that obtains optical tomographic images at high speed without changing the optical path length of a reference light beam. This SS-OCT optical tomograph utilizes a light source that periodically sweeps the frequency of a laser beam. Reflected light beams of each wavelength are caused to interfere with reference light beams of each wavelength. Temporal waveforms of signals corresponding to the temporal variations in the frequency of the laser beam are measured, and a computer administers Fourier transform on the obtained spectral interference intensity signals, to obtain a tomographic image.
It is desirable to employ a measuring light beam having a wide spectral width, in order to improve spatial resolution in the aforementioned TD-OCT measurement, SS-OCT measurement, and SD-OCT measurement (refer to Japanese Unexamined Patent Publication No. 2002-214125). Japanese Unexamined Patent Publication No. 2002-214125 discloses a method that widens the spectral width of the measuring light beam, in which a plurality of light sources that each emit light beams having a different spectral band are used, and an optical integrator integrates the light beams emitted from the plurality of light sources, to obtain a single light beam.
However, there is demand for a system that enables obtainment of tomographic images having favorable image quality that employs a light source having a simple structure, as opposed to the combination of light sources as disclosed in Japanese Unexamined Patent Publication No. 2002-214125, which requires stringent control.